Solid ink or phase change ink printers conventionally receive ink in a solid form, either as pellets or as ink sticks. The solid ink pellets or ink sticks are placed in a feed chute and a feed mechanism delivers the solid ink to a heater assembly. Solid ink sticks are either gravity fed, belt driven or urged by a spring through the feed chute toward a heater plate in the heater assembly. The heater plate melts the solid ink impinging on the plate into a liquid that is delivered to a print head for jetting onto a recording medium. U.S. Pat. No. 5,734,402 for a Solid Ink Feed System, issued Mar. 31, 1998 to Rousseau et al.; and U.S. Pat. No. 5,861,903 for an Ink Feed System, issued Jan. 19, 1999 to Crawford et al. describe exemplary systems for delivering solid ink sticks into a phase change ink printer.
Phase change inks for color printing typically comprise a phase change ink carrier composition which is combined with a phase change ink compatible colorant. A color printer typically uses four colors of ink (yellow, cyan, magenta, and black). These colored inks can be formed by using a single dye or a mixture of dyes. For example, magenta can be obtained by using a mixture of Solvent Red Dyes or a composite black can be obtained by mixing several dyes. Suitable carrier materials can include paraffins, microcrystalline waxes, polyethylene waxes, ester waxes, fatty acids and other waxy materials, fatty amide containing materials, sulfonamide materials, resinous materials made from different natural sources (tall oil rosins and rosin esters, for example), and many synthetic resins, oligomers, polymers, and copolymers.
Due to the soft, waxy nature of an ink stick body, ink sticks may be easily damaged during handling. Therefore, it is common to package ink sticks in containers after manufacturing. Typically, ink sticks are placed in a tray, such as a thermoformed polystyrene tray having one or more cavities, or compartments, for holding a plurality of ink sticks. The trays typically have a flange that surrounds the perimeter of the package and intermediate flanges that separate each compartment of the tray. A thermoplastic film or laminate may then be positioned over tray and heat sealed to the flanges to hermetically enclose the ink sticks within each compartment.
To heat seal the film to the tray, a heated bar engages the film to compress it against the flanges of the tray. In so doing, heat transfers from the heated bar to the film, through the thickness of the film, and to the flange of the tray. The resulting heat and compression causes the contacting surfaces of the film and flange to become molten and to intermix with one another. The heating bar is then removed to allow the sealed area to cool and form a sealed bond.
One of the chief difficulties encountered when applying films to containers is the maintenance of consistent seal strength for the containers produced. This problem is exacerbated by the increasing complexity of ink stick shapes which results in increased size for the compartments for containing the ink sticks in the trays. Because it may not be practical or economical to keep increasing the overall size of the containers to accommodate the ink sticks, the increasing size of the compartments in the trays may limit the areas upon which to seal the film. Therefore, the reduction in the potential sealable area may lead to a reduction in seal strength in these reduced areas resulting in the film possibly peeling off of the trays. Not only does this have an aesthetically unpleasing appearance, but the ink sticks in the trays may be exposed to contaminants such as excessive moisture.
Prior systems typically used a substantially flat heated bar to press the sealing film against the flange of the tray. This method has the disadvantage of heating the sealing film directly over the compartments and, consequently, the contents of the compartments which, in the solid ink sticks, may damage and/or otherwise melt the contents of the container. To avoid this, some previously known sealing bars were equipped with thin strips, or ridges, that were shaped and positioned on the bar to correspond to the lip of the compartment on the tray. In the case of a multi-compartment container, the heated bar 2 may have a plurality of ridges 4 positioned on the bar to contact the areas of the flange of the tray surrounding each compartment (See FIG. 1). The ridges 4 ensure that the maximum heat and pressure would be applied to only the areas surrounding each container thereby lessening the amount of heat directly over the compartments.
The use of ridges, however, presented additional problems to sealing the film onto the tray. For instance, as can be seen in FIG. 1, the ridges 4 may leave gaps 8 in the contact surface which may result in unsealed areas in the flange areas between the sealed compartments. Typically, the sealed trays containing the ink sticks would be scored in the flange areas between the tray compartments to facilitate separation of the compartments from the tray. The scoring may increase the likelihood of seal failure and unintentional peeling of the film in these areas due to the unsealed gaps. In addition, any misalignment of the ridges of the seal plate with respect to the compartments of the tray during sealing may cause a misalignment and misplacement of the actual seal created between the film and the tray resulting in gaps or breaks in the seal.